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IEEE 802.11 WLANIEEE 802.11 WLAN

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IEEE 802.11 Protocol StackIEEE 802.11 Protocol Stack

For centralized contention-free channel access

For distributed contention-based channel access

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Possible Network TopologiesPossible Network Topologies

BSS mode ESS mode

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802.11: Channels, association802.11: Channels, association

802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference possible: channel can be same as

that chosen by neighboring AP!host: must associate with an AP

scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address

selects AP to associate withmay perform authenticationwill typically run DHCP to get IP address in AP’s

subnet

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Distributed Coordination Function Distributed Coordination Function (DCF)(DCF)

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Binary Exponential BackoffBinary Exponential Backoff

Backoff Counter is randomly selected from [0,CW],where CW is contention window

For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax)CW 2 (CW+1)-1

If successful transmission,CW CWmin

Reduces the collision probability

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Virtual Carrier SensingVirtual Carrier Sensing

Any node hearing RTS or CTS sets up their NAV (network allocation vector) until end of ACK.

NAV set -> node silent (act as if carrier busy).

A B C DRTSRTSCTSDATAACK

E

CTSDATA

ACK

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802.11 Timeline802.11 Timeline

If carrier busy (physical or virtual), schedule transmission after a random backoff when carrier is free.

Average backoff interval is doubled for each failed attempt.

t

SIFS

DIFSACK

Defer access

Nodes that hear transmitter

Receiver

TransmitterDATA

Randombackoff

RTS

CTS

SIFS SIFS

NAV (RTS)NAV (CTS)

DIFS

Nodes that hear receiver Another

transfer

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RTS/CTS Mechanism (Optional)RTS/CTS Mechanism (Optional) RTC/CTS solves HTP But, non-negligible

overhead If frame size > RTSthreshhold,

• RTS-CTS-DATA-ACK Otherwise,

• DATA-ACK

802.11b

tslot 20usec

SIFS 10usec

PIFS SIFS + tslot

DIFS SIFS + 2*tslot

EIFS > DIFS

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Collisions are not completely avoidedCollisions are not completely avoidedin IEEE 802.11 !!in IEEE 802.11 !!

H does not sense any signal during D’s DATA tx H may transmit Collision in E’s reception

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Energy Conservation: PowerEnergy Conservation: Powercontrolcontrol Power control has two potential benefit

Reduced interference & increased spatial reuseEnergy saving

If C reduces transmit power, it can still communicate with DReduces energy consumption at node CAllows B to receive A’s transmission (spatial reuse)

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framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: addressing802.11 frame: addressing

Address 2: MAC addressof wireless host or AP transmitting this frame

Address 1: MAC addressof wireless host or AP to receive this frame

Address 3: MAC addressof router interface to which AP is attached

Address 4: used only in ad hoc mode

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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

802.11 frame: addressing802.11 frame: addressing

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framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

802.11 frame: more802.11 frame: more

duration of reserved transmission time (RTS/CTS)

frame seq #(for reliable ARQ)

frame type(RTS, CTS, ACK, data)

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The Other IEEE 802.11 EffortsThe Other IEEE 802.11 Efforts 802.11e

Provides QoS support by differentiating traffic streams Applicable to 802.11 PHY a, b, and g

802.11h Supplementary to MAC layer so as to comply with European

regulations for 5 GHz WLAN 802.11i

Security enhancement 802.11n

Enhancement for higher throughput (> 100 Mbps ) Decrease overhead within 802.11 protocol

• Packet preamble, CW, ACK, IFS parameters 802.11r

Speed up handoff between APs (Fast BSS-Transition) Important for VoWLAN

802.11s Support mesh networks